Structure‐performance relationships in the propene polymerization behaviour of a number of silicon bridged bisindenyl zirconocenes show a rational pattern. Prognosis of the polymerization behaviour of different type structures, however, often fails, which is demonstrated by three examples of new zirconocenes. These include two bisindenyl complexes with two‐membered silicon bridges and one isospecific bridged fluorenyl cyclopentadienyl species. It is thus demonstrated that the scope of a “rational catalyst design” in the field of metallocene catalysts is still limited.
Synthesis of the Macrolide Antibiotic (-)-A26771B Using Photolactonization as a Key Reaction and Computer Simulation as an Effective Aid in OptimizationThe title compound has been synthesized in a sequence of 21 steps starting from phenol. The cyclic skeleton has been built up in the earlier part of the synthesis by photolactonization. This reaction supplies C(6) with a functionality, not present in the target structure, but useful for oxygenation at C(4) and generation of the stereogenic center C(5). The BarlonlMcCombie reaction provides a product deoxygenated at C(6), but only after an adjacent oxirane ring has been opened at the cost of an increase in the number of overall steps. X-Ray structural information and computational modelling of appropriate molecules proved to be essential to selection of optimal conditions for various transformations.
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